In a large bowl, cream together the butter, white sugar, and brown sugar. Mix in eggs and 1 teaspoon vanilla until fluffy. Sift together the flour, salt and baking powder; stir into the creamed mixture. Stir in the coconut, dates, sultanas and zucchini. Spread the mixture into the prepared tin.
Bake in preheated oven for 35 to 40 minutes. To make the icing, mix together melted butter, milk, 1 teaspoon vanilla, cinnamon, and icing sugar. Drizzle icing over the slice while still warm. Sprinkle chopped nuts over icing. Cut into slices when cool.